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doi:10.1016/j.wasman.2005.12.012
Technical paper
Thermal behaviour of chrome shavings and of sludges recovered after digestion of tanned solid wastes with calcium hydroxide
S. Tahiri
a,¤, A. Albizane
b, A. Messaoudi
b, M. Azzi
c, J. Bennazha
b, S. Alami Younssi
b, M. Bouhria
ba Faculté des Sciences d’El Jadida, Université Chouaib Doukkali, Département de Chimie, B.P.20, El Jadida, Maroc
b Faculté des Sciences et Techniques, Université Hassan II-Mohammedia, B.P.146, Avenue Hassan II, C.P.20650, Mohammedia, Maroc
c Faculté des Sciences Aïn Chock, Université Hassan II, Km.8, Route El Jadida, B.P.5366, Maârif, Casablanca, Maroc Accepted 6 December 2005
Abstract
The thermal behaviour of chrome shavings and of sludges recovered after digestion of tanned wastes with Ca(OH)2 was studied.
Ashes obtained after incineration of wastes at various temperatures were analysed by X-ray diVraction and EDX method. The main crystallized phases present in the ash obtained at 600 °C are Cr2O3 and NaCl. The diVractograms revealed an increase in the intensities of the chromium oxide peaks and a very notable decrease of the amount of sodium chloride at 1100 °C. EDX analysis revealed a total disappearance of the chlorine peak at this temperature. Scanning electron micrographs show that the waste lost its Wbrous aspect when the temperature increases. Formation of aggregates was noted after 550 °C. Combustion of organic matters and decarbonation phe- nomenon are the main stages observed on GTA and DTA curves of sludges. These phenomena are, respectively, exothermic and endo- thermic. The diVractogram of sludges recorded at 550 °C, in the presence of a constant oxygen surplus, revealed the presence of CaCrO4 and CaCO3.
© 2006 Elsevier Ltd. All rights reserved.
1. Introduction
The operations involved in the transformation of hides into leather generate both liquid (wastewater) and solid (tanned and untanned solid waste) pollution loads at various processing stages. Through the application of clean technologies (water savings, recycling of most pol- lutant baths) and solid wastes revaluation techniques, the mass balance for tanneries can be signiWcantly improved (Panswad et al., 1995; Carré et al., 1983; Aloy et al., 1976).
The production of chromium-containing solid waste (including chrome shavings and tanned splits) in a tannery has been recognized as a problem for many years, but recently pressure from environmental authorities has given
the problem increasing urgency. Historically, shavings, trimmings and splits from the chrome tanning of hides and skins have been disposed of in landWlls. Increased local restrictions on land disposal, recent increases in the costs of land disposal and decreases in the number of disposal sites have combined to spur research into alternative treat- ments for this material. The chromium in these solid wastes exists in the non-toxic +3 oxidation state, but there is growing concern that chromium could possibly be con- verted to a toxic state. Many scientiWc groups have ori- ented their research to Wnd a process to recycle and treat these wastes (Tahiri et al., 2001a,b, 2002, 2003, 2004; Tay- lor et al., 1998; Cabeza et al., 1998; Brown et al., 1996;
Bataille et al., 1983).
Many investigators have recovered chromium by wet air oxidation (Okamura and Shirai, 1976), peroxide treatment (Cot and Gratacos, 1975) and incineration at a variety of temperatures (Okamura et al., 1981). Under precise conditions, i.e., a temperature between 450 and 600 °C, an
* Corresponding author.
E-mail address: [email protected] (S. Tahiri).
alkaline pH ensured by the addition of lime or sodium car- bonate, and in the presence of a constant oxygen surplus, it is possible to transform all of the chromium present in tanned wastes into hexavalent chromium. Concentrated solutions of sodium bichromate are regenerated by simple leaching with acidiWed water. The residual ashes are com- pletely white, which is the sign of a total absence of hexava- lent or trivalent chromium. Another possibility consists of opting for a higher temperature (850–1000 °C) limiting to the maximum extent the conversion of trivalent chromium into hexavalent chromium (Carré et al., 1983).
In this work, the behaviour of chrome shavings vs. tem- perature was studied. The obtained ashes were analysed by X-ray diVraction and EDX method. The raw wastes and the ashes obtained at various temperatures were observed using scanning electron microscopic technique. The ther- mal behaviour of sludges, obtained after digestion of chrome shavings with Ca(OH)2, was also studied using thermogravimetry analysis (GTA) and diVerential thermal analysis (DTA).
2. Materials and methods
2.1. Materials
2.1.1. Chrome shavings
The small pieces of leather are shaved oV when the thick- ness of wet blues is rendered uniform by a bladed cylinder.
The wet blue is the wet chrome tanned leather without dressing. The structure of wet blue and chrome shavings is presented in Fig. 1. The samples were collected from a tan- nery and stored in sacks of plastic at room temperature.
2.1.2. Sludges
This solid residue was obtained according to the follow- ing process (Fig. 2): to 10 g of chrome shavings, we added 2 g of calcium hydroxide and 100 mL of water. The mixture was heated for 15 min, and the solution was Wltered. The proteins are in the aqueous phase and the metallic salts in the solid phase or sludges (Bataille et al., 1983).
2.2. Analysis
Chrome shavings were analysed for: ash, chromium oxide, fat, dermal substance and pH of soluble matter. The physical and chemical characteristics of wastes were ana- lysed according to standard methods (Dutel, 1984).
An ICP-OES Perkin–Elmer Optima 3200 RL (Connecti- cut, USA) with argon as plasmogen gas was used for the elemental analysis of chrome shavings using, for each ele- ment determined, the most sensitive wavelength.
Chromium was determined in ashes and sludges using atomic absorption spectrometry (UNICAM AAS). Mea- surement of the chromium concentration was carried out in the nitrous oxide/acetylene Xame at 357.9 nm. Determina-
Fig. 1. Structure of wet blue and chrome shavings.
Fig. 2. Alkaline digestion of chrome shavings.
Heating to boiling point H2O 100mL
+ Ca(OH)2 2g
Lyophilisation
Separation
Raw matter (Solid proteins)
Liquid Solid residue (Sludge):
chromium hydroxide and no digested wastes Reactor
Chrome shavings (10g)
tion of calcium in ashes was carried out using the same apparatus in air/acetylene Xame at 422.7 nm.
The sludges obtained, after digestion of chrome shavings with Ca (OH)2, were analysed. The moisture, the content of chromium and the percentage of mineral matter were deter- mined.
The X-ray diVraction patterns of the ashes obtained at various temperatures (600, 750, 900 and 1100 °C) were recorded with SIEMENS D5000 with Cu K radiation (CuD1.5406 Å).
Observations of structure and morphology of chrome shavings and ashes were made using a scanning electron microscope (HITACHI S-4500).
The EDX patterns of ashes were recorded with delta KEVEX apparatus.
Thermal analysis experiments, GTA and DTA, were car- ried out, respectively, in TG 85–1000 °C SETARAM and NETZSCH DiVerential Thermoanalyzer DTA 404. Gravi- metric thermal analysis (GTA) is used to study chemical, physical or physico-chemical phenomena that occur due to the eVect of temperature, by looking at changes in mass of the sample. Sludges (22.4 mg) were heated at a rate of 300 °C per h to 1000 °C. The sensitivity of the technique is 0.1 mg. DiVerential thermal analysis (DTA) is used to study endothermic and exothermic phenomena that occur during pyrolysis of the sample. DTA gives information on allo- tropic transformation of the sample, decomposition, dehy- dration and changes of state. The principle consists of measuring the diVerence in temperature between the sample and a reference while both are heated together. Any change in physical or chemical state can be followed by the absorp- tion or the emission of heat. Al2O3 was used as the reference material. The experiment was performed in an atmosphere of oxygen.
2.3. Incineration of chrome shavings
A Nabertherm (Germany) Furnace was employed for incineration of wastes. The chrome shavings were heated using the following program: heat the sample at an appro- priate rate (50 °C/30 min) to 50 °C and hold for 2 h. The sample was weighed after cooling in a dessicator and then returned to the furnace, where the temperature was increased another 50 °C, held for another 2 h and again weighed. Heat treatments were continued in 50 °C incre- ments until the temperature reached 1100 °C.
3. Results and discussion
3.1. Physico-chemical characters of chrome shavings
It is useful to analyse these wastes in order to have an idea of their physico-chemical characteristics. The chrome shavings, used as raw material to run the experiments, were analysed for pH, ash, fat, proteins and chromium oxide.
The results are presented in Table 1. As can be seen, the analysis indicates that the waste has a signiWcant percent-
age of proteins (78.6%). The percentage of chromium oxide is about 4.4%. The concentration of diVerent elements are expressed in g/kg (Table 2), in the dry solid wastes and obtained through the method of ICP-OES. Chromium is not the only compound present in tannery wastes. As leather undergoes several diVerent treatments, other chemical compounds could be present. Results show that chrome shavings have a signiWcant concentration of sodium (93.9 g/kg) and calcium (6.9 g/kg). This can be explained by the use of some chemical products, such as sodium chloride and lime, in tanneries.
3.2. Scanning electron micrographs of chrome shavings and of ashes obtained at various temperatures
The scanning electron microscopic method is a good technique for showing the structure and morphology of solids. The resulting micrographs revealed that the studied wastes have a highly organized structure in the form of Wbers. The diameter of these Wbers is about 100 nm. The micrography of waste heated at various temperatures show that the material (as would be expected) lost its Wbrous character when the temperature increases. For temperatures ranging from 25 to 200 °C, obtained micro- graphs show that the shrinkage of Wbers is more signiWcant
Table 1
Physico-chemical characters of chrome shavings
a % Based on weight of dry waste.
b 5 g/100 mL (waste/water).
Parameter Value
Moisture (%) 22.3
Mineral matter (%)a 12.9
Chromium oxide Cr2O3 (%)a 4.4
Fat (%)a 2.5
Dermal substance (%)a 78.6
pH of soluble matterb 3.6
Table 2
ICP-OES multielement determination in chrome shavings
Element Concentration (g/kg)
Na 93.86
Ca 6.91
Mg 0.60
Fe 0.80
Mo 0.008
Al 0.43
Zn 0.47
Ti 0.02
V 0.02
Cd <LOD
Co <LOD
Cu 0.05
Li 0.05
Ni 0.008
Cr 30.10
Mn 0.008
Pb 0.05
when the temperature increases. This phenomenon is strongly observed after 100 °C (shrinkage temperature of wet-blue). Formation of aggregates was noted after 550 °C (Fig. 3).
3.3. Yields of the products isolated using Ca(OH)2
Solid residue and protein yields were given (Table 3).
The results obtained revealed that:
– the amount of dry residue is about 4 g (solid residue/dry wastesD50.6%),
– the amount of protein product is about 6 g (proteins/dry wastesD77.7%).
3.4. Sludge analysis
The results obtained are presented in Table 4. The results show that the alkaline digestion using Ca(OH)2 allows a
total recovery of chromium (»100%) in the form of sludges.
The percentage of chromium reported in the mass of slud- ges, with 14.6% moisture, is about 6%. Results show also that the Ca/Cr ratios are, respectively, 4% and 5.2% for weight and molar percentage. The weight percentages of chromium and calcium in mineral matter are, respectively, 18% and 72.2%.
3.5. Behaviour of chrome shavings during the incineration The very low density of chrome shavings and the limit of thermal analysis (DTA and GTA) application obliged the use of the calcination to study the waste behaviour with temperature. Table 5 indicates the losses of mass observed
Fig. 3. Scanning electron micrograph of chrome shavings treated at various temperatures.
Table 3
Yields of the isolated products
Dry matter (protein product) (g) 6.0
Solid residue (sludge) (g) 3.9
Dry matter/raw wastes (%) 60.4
Dry matter/dry wastes (%) 77.7
Solid residue/raw wastes (%) 39.3
Solid residue/dry wastes (%) 50.6
Table 4 Sludges analyses
a Based on weight of dry cake.
b Based on weight of mineral matter.
Parameter Value
Moisture (%) 14.6
Mineral matter (%)a 72.2
Organic matter (%)a 27.8
Crb (%) 18.0
Cab (%) 72.2
Ca/Cr (weight percentage) (%) 4.0
Ca/Cr (molar percentage) (%) 5.2
Recovered Cr/total Cr (%) »100
Cr/cake (w/w) (%) 6.0
between the temperatures that represent the main stages of transformation. The signiWcant loss (42–43%) observed between 25 and 105 °C, corresponds to the evaporation of moisture content. This loss is an endothermic phenomenon.
A signiWcant loss (49.9%) was noted between 105 and 550 °C, which corresponds to the combustion of great amount of organic matter. A strong release of fume was noted at the range of temperature corresponding to this exothermic phenomenon. The lower loss (0.4%) observed between 900 and 1100 °C, corresponds probably to the vaporization of sodium chloride. The study carried out by Bacou (1997) conWrms this hypothesis: after incineration of sludges obtained from physico-chemical treatment of tan- nery eZuents, the salts found in the chimney of the inciner- ator were analysed. The results showed the presence of signiWcant amounts of sodium chloride.
Fig. 4 shows the evolution of chromium oxide content of waste vs. incineration temperature. The content of Cr2O3 increases with temperature. The chromium oxide percent can reach 35.8% for temperatures ranging from 550 to 1100 °C.
3.6. X-ray diVraction and EDX analysis of ashes
X-ray diVraction (XRD) can be used to determine if crystalline phases were formed, and then to identify these phases. This method allows following the mineralogical composition of wastes with temperature. The diVracto- grams recorded are presented in Fig. 5. The experimental diagrams were analysed according to the values given by
ASTM Wles. The peaks were identiWed by their position on the diagram (angle 2) and their height in coups s. Results show that the main crystallized phases present in the ash obtained at 600 °C are chromium oxide Cr2O3 and sodium chloride NaCl. The results obtained revealed also an increase in the intensities of the chromium oxide peaks and a very notable decrease of the amount of sodium chloride at high temperature (1100 °C). This decrease is due to the vaporization of the salt. EDX analysis of the ashes obtained at 750, 900 and 1100 °C revealed a total disap- pearance of the peak corresponding to the chlorine at 1100 °C. A good correlation between the results of X-ray diVraction and of EDX analysis was noted.
3.7. Thermal analysis of sludges recovered after alkaline digestion of chrome shavings with Ca(OH)2
GTA and DTA curves of sludges obtained from ambient temperature to 1000 °C, using a heating rate of approxi- mately 300 °C h¡1, are shown in Fig. 6. The losses of mass observed between temperatures are presented in Table 6.
Table 5
Losses of chrome shavings mass (%) observed between temperatures Temperature To 105 °C To 550 °C To 900 °C To 1100 °C
From 25 °C 42.0–43.0 92.4 92.6 92.9
From 105 °C – 49.9 50.1 50.4
From 550 °C – – 0.2 0.5
From 900 °C – – – 0.4
Fig. 5. X-ray diVraction of ashes. (+) Cr2O3, (¤) NaCl.
Fig. 4. Evolution of chromium oxide content vs. temperature.
0 5 10 15 20 25 30 35 40 45
0 200 400 600 800 1000 1200
Temperature (˚C) Cr2O3 (%)
Chrome shavings Tanned s plits
A Wrst period of decreasing mass (7.6% mass loss) was registered before 200 °C, and corresponds to the evapora- tion of moisture. The corresponding phenomenon is endo- thermic. A second stage of mass decrease (9.9%) was observed between 200 and 550 °C, and corresponds to the evaporation of organic matters content. The stage can be
separated in two sub-periods. The DTA peaks of this stage were registered at 317 and 500 °C. The corresponding phe- nomenon is exothermic. Another signiWcant mass decrease (20.8%) was observed between 550 and 750 °C and the cor- responding DTA peak was registered at 730 °C. This endo- thermic stage has been attributed to the decarbonation phenomenon:
CaCO3!CaOCCO2
The temperature from which the samples are considered completely mineralised is Wxed at 550 °C by the interna- tional norms (Bacou, 1997). In order to identify the crystal- line phases formed at this temperature, in the presence of a
Fig. 6. GTA and DTA curves of sludges.
Table 6
Losses of sludges mass (%) observed between temperatures
Temperature To 200 °C To 550 °C To 750 °C
From 25 °C 7.6 17.5 38.4
From 200 °C – 9.9 30.8
From 550 °C – – 20.8
Fig. 7. X-ray diVraction of ash obtained after incineration of sludges at 550 °C. (¤) CaCO3, (䊐) CaCrO4.
constant oxygen surplus, we have analysed the obtained ash by X-ray diVraction. The diVractogram revealed the pres- ence of calcium chromate CaCrO4 and of calcite CaCO3. The peak noted at 2D25.5 can be attributed, probably to 100 of CaCrO4· H2O (Fig. 7).
After oxidation of chromium (III), the obtained ashes could be used to recover chromium (VI) by simple leaching with acidiWed water. The chromium present in the sludges in the form of Cr(OH)3 is converted to the chromium (VI) in the oxygen-rich atmosphere of the incineration process. This conversion is accelerated in the presence of calcium com- pounds. The high calcium content of the sludges is a positive factor for increasing the percentage of chromium oxidation.
The presence of alkalinity source (NaOH) is also positive to increase the oxidation yield (Beccari et al., 1987, 1992).
4. Conclusions
Chrome shavings have a signiWcant amount of proteins (78.6–75.2%) and of chromium oxide (4.4–4.3%). In this work, two treatment processes were used:
– Incineration of the waste product without digestion to recover chromium and caloriWc value.
– Alkaline digestion with lime, to recover protein frac- tions, followed by incineration of sludges to recover chromium.
The studies carried out have evidenced the thermal behaviour of tanned wastes and of sludges recovered after their digestion with Ca(OH)2. The techniques employed in this work allow following the chemical composition of wastes with temperature and the phenomena which occur during incineration of the samples. The main crystallized phases present in the ash obtained after incineration of chrome shavings at 600 °C are Cr2O3 and NaCl. The diVractograms revealed an increase in the intensities of the chromium oxide peaks and a very notable decrease of the amount of sodium chloride at 1100 °C. Micrographs show that the waste lost its Wbrous characteristic when the tem- perature increases. After extraction of protein product from wastes with lime, the sludge that remains is further incinerated. Combustion of organic matter and decarbon- ation phenomenon are the main stages observed on the GTA and DTA curves of sludges. These phenomena are, respectively, exothermic and endothermic. The diVracto- gram of sludges recorded at 550 °C, in the presence of a constant oxygen surplus, revealed the presence of CaCrO4 and CaCO3.
The ashes obtained after incineration of chrome shav- ings have a signiWcant amount of chromium Cr III (35.8%
of Cr2O3). In order to revalue these ashes and avoid pollu- tion transfer resulting in soil pollution (Wnal disposal), these products will be used in several industries, such as cementry
and pottery. Treatment and reuse of the concentrated solu- tions regenerated by leaching with acidiWed water of ashes of sludge, recovered after digestion, is also one of the objec- tives of our later works.
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